Mexico Could Tap More Oil, Store It as Loan Collateral S. FRED SINGER, S. Fred Singer is Eminent Scholar at George Mason University in Fairfax, Va., on leave from the University of Virginia. He formerly served as deputy assistant secretary of the Interior. DR, NARANJO, El Universal, Mexico City

Japan Has Downtrodden Minorities, Too GEORGE A. De VOS, George A. De Vos is the former director of the Center for Japanese Studies at UC Berkeley. He is a co-author with Hiroshi Wagatsuma of "Heritage of Endurance, a Study of Family and Delinquency in Urban Japan" (UC Press, 1985).